1. Field of the Invention
The invention relates in general to a method and apparatus of electrolytically microfinishing metallic workpieces. More particularly, this invention has specific reference to the application of electrolytic machining to a microfinishing process with a view to decrease considerably, the cycle time of finishing the workpiece to be treated while reducing appreciably, the cost of the microfinishing operation for a given degree of precision.
2. Description of the Related Art
There are two well know methods of machining work pieces using electrolytic cutting.
The first is electrochemical machining (ECM) which is done by using a cathode having the shape of the part to be machined. The shape of the cathode is transferred to the workpiece (anode) by deplating, similar to electroplating in accordance with Faraday's Laws. In an electrolytic conductive solution, electrical current is applied to the workpiece to de-plate the material while an electrolyte is pumped between the cathode and the workpiece (anode) and prevents the deplated material from plating out on the cathode. For the purpose of this document, the electrolytic process referred to in the various discussions can include both electrical discharge machining and electrochemical machining although they have recognized distinct differences.
The second is electrochemical grinding (ECG). In this process, the cathode is the grinding wheel. As the electric current flows between the workpiece and the wheel, the material removed by electrolysis is carried off by the abrasives in the rotating wheel. The wheel is made of a conductive material into which abrasive particles have been embedded and touches the workpiece very lightly. The tool and workpiece are connected to a direct current source. The electrolyte is applied onto the grinding wheel near the workpiece in a manner that will result in the wheel carrying it into the cut. This brings about electrochemical action, molecular decomposition or deplating of the workpiece.
Electrolytic cutting has advantages over mechanical and heating methods of cutting metals. Electrolytic methods provide cuts that are free from mechanical or thermal strain and thus do not disturb the grain structure of the cut material. Electrolytic cutting methods also avoid the burrs and jagged edges and the avoidance of metal as dust or other small particles in mechanical cutting or as vapor that is evaporated or burned away where a cutting flame is used.
The advantages and techniques involved in removing particles of electrically conductive materials by electrolytic erosion (often called electrochemical decomposition) are also well established in the industry. Both of these electrolytic stock removal processes have been found useful in grinding, as well as shaping operations, particularly when the workpiece materials are extremely hard.
Electrolytic grinding briefly consists of bringing a workpiece against the face of a rotating metal bonded grinding wheel under conditions where a low voltage direct current passes through an electrolyte between the workpiece and a wheel during the operation so as to remove material from the workpiece by electrolytic action. Electrochemical grinding (ECG) is principally used for specialized areas, for example, the grinding of flat surfaces or cutting form surfaces with preformed grinding wheels. In electrolytic grinding, the abrasive particles serve to remove non-conductive films which may form on the surface of the material of the workpiece which is being ground and may serve also to provide abrasive cutting action. By this conjoint electrolytic and abrasive action, the cutting speed is very much enhanced as compared to that obtained by surface grinding alone. On the other hand, it is also possible to rely almost entirely upon electrolytic action thereby reducing the rate of wear on the abrasive particles to a minimum. Because metal removal is largely brought about by non-mechanical action, only about 10% of the conventional grinding wheel pressure is required, corresponding to the fact that only about 10% of the material is removed by the abrasive action of the conductive grinding wheel. Most of the metal removal is brought about by the electrochemical action. The metal removal rate is largely governed by the amount of electric current and electrolyte applied, regardless of the material's hardness. It is possible with good process controls to achieve surface finishes as low as Ra=0.1 μm using electrolytic grinding techniques.
In spite of the many advantages, electrolytic decomposition or grinding may provide little or no success that has been achieved in applying the principle of electrolytic erosion to the microfinishing process.
Microfinishing is a somewhat misunderstood process. In the prior art it is at times referred to as superfinishing, lapping, honing, mirror finishing, fine grinding, or just plain finishing. Microfinishing, as used herein is intended to encompass all of the above-listed prior art terminology. Accordingly, microfinishing is a surface finishing process that is performed after rough, medium machining, or fine grinding of the surface of a workpiece, such as a previously machined workpiece or medical prosthesis. Microfinishing is typically performed using a machine that brings some form of rotating and/or oscillating abrasive material into contact with the workpiece while the workpiece is rotated. The abrasive material applied subsequent to a previous machining operation removes any defects, like surface imperfections, and is used to obtain particularly accurate geometrical characteristics of the surface including exceptional surface finishes. Microfinishing is a low temperature machining process which combines the motion of the workpiece and the motion of a bonded abrasive “stone” or “tape” to generate both a geometrically accurate form and specific surface finish. The surface characteristics of the generated surface are typically the function of the specific stone or tape grit size used. The abrasive, under an extremely low and relatively constant force, will produce the repeatable surface characteristics or qualities required generating little or no heat. One advantage of microfinishing is the elimination of the amorphous layer after grinding. The amorphous layer or recast layer is a product of any heat generating process. In machining or grinding with any calculated feed method, the ability of the tool to cut is not the only determining factor for the feed rate. The incoming geometry and the surface finish are also an integral part of the equation. These are at best all variables. In many cases the derived feed rate may be extremely small. In the true microfinishing process, the stone determines the rate and duration of feed depending upon the incoming conditions and geometry of the workpiece. Once the stone has progressed through the soft amorphous layer, however thick, and has reached the solid base material, the stock removal rate will dramatically reduce to the point when the stone glazes. Once this occurs, the geometry generating portion of the process will be complete and the second stage begins. During the second stage, a measurable amount of stock will no longer be removed, but the glazed stone will act as a polishing tool and create the required finish. The specific stone and grit size is picked for its ability to remove the soft amorphous layer, produce the desired geometry and also the required finish. This process induces no metallurgical alteration and provides a clean, burr-free workpiece.
As set forth above, in the first stage, the smooth microfinish tool of bonded abrasive grain contacts the rough surface of the workpiece. The rough surface of the workpiece removes the glazed portion of the stone from the previous operation and aggressive cutting of the amorphous layer begins. During this second stage, the rough abrasive tool surface continues to cut and wear. As the geometry and finish of the workpiece improves, the abrasive surface of the stone also becomes smoother and starts to re-glaze. This results in a lower feed rate and consequently decreases wear. Finally, in the third stage the workpiece and abrasive tool surfaces are both extremely smooth. The ability of the stone to cut is minimal and the final finishing stage is achieved. The cutting speed and contact pressure of the microfinish process are so small that heating of the workpiece surface remains well below a detrimental limit thereby avoiding the generation of a new amorphous layer on the workpiece.
Microfinishing can generate flat, spherical, or cylindrical surfaces. There are three basic types of spherical shapes that are applicable to the microfinish process. These are complete spheres, internal or concave spheres, and partial conical shapes. Flat and complete spherical surfaces are the easiest shapes to achieve a very accurate, uniform geometry and finish, while the partial/conical shape proves to be the most difficult. With carefully selected angular approaches and properly selected stones, results of the relative same degree of accuracies for full spheres can be achieved. Outside diameters on cylinders, shafts, piston and journals, inside diameters and bores are microfinished by using an area contact instead of a line contact as by conventional grinding. This improves the roundness, surface finish and eliminates leads.
Since stock removal is obtained at relatively low cutting forces and speeds, as compared to grinding, this leads to a surface characterized by higher compressive stresses, and the elimination of damage to the finish surface by excessive heat. As discussed above, a first step is used in order to optimize the stock removal and geometry process and then followed by a secondary step in order to create the surface structure required. This second step is characterized by a small stock removal. Abrasive grit sizes can range from approximately 320 to 1500, and include silicon carbide, aluminum oxide, C.B.N. (cubic, boron nitride) and diamond bonded abrasives. Where required, prior art production microfinishing processes achieve cylindrical surfaces with Ra as low as 0.05 μin and with roundness of 1 μin (0.025 μm) Ra is defined as the arithmetical average profile deviation of the surface irregularities with respect to a hypothetical perfect surface established by an arithmetical average line (see U.S. Pat. No. 6,222,628).
In comparison, fine grinding without the use of electrolytic action utilizes tools made of similar abrasives i.e., silicon nitrate, aluminum oxide, C.B.N. or diamond in grit size from 280 to 1200. Since fine grinding involves rotation by either chucking or centerless driving, during the relative rotational motions of the workpiece and wheel, combined with the corresponding contact angle, generates the required surface form i.e., flat, spherical (concave or convex) with a characteristic cross hatch pattern as shown in prior art FIG. 1. The requirements for the same low cutting forces are utilized in the fine grinding process. Surface speeds generally are higher than microfinishing, especially when using C.B.N. or diamond tools. The feed systems utilized are normally very sensitive so as to not force the grinding tool into the work area and develop excessive heat. N.C. feeds are generally not sensitive enough for many applications, and air feed with hydraulic damping is usually preferred. The surface produced by grinding is more or less wavy. It is these wavy surface defects that are eliminated with microfinishing. Temperatures of up to 1100° C. can develop in the line of contact between the workpiece and grinding wheel, generating an amorphous layer, also known as “soft skin”. This amorphous layer substantially reduces the load carrying capacity and therefore has to be eliminated by microfinishing in order to take advantage of the maximum performance characteristics of the metallic workpiece. Production fine grinding processes with carefully controlled process parameters can achieve flat surfaces with Ra's as low as 4.0 μin (0.1 μm).
Increased demands for more accurate processing of newly developed materials, shorter lead time, more accurate dimensional as well as surface characteristics has resulted in the investigation of new machine processing techniques. Of course these increased demands are always expected to be developed at lower costs for automotive, diesel engine, aerospace, hydraulic, medical devices and many other precision part manufacturers. In response to these demands there have been some attempts to obtain more accurate dimensional surfaces as well as surface finishing characteristics. For example, U.S. Pat. Nos. 4,140,598 and 4,328,083 to Kimoto et al. disclose a mirror finishing process which combines electrolytic machining with an abrasive tool. In Kimoto et al., abrasive powder is mixed into the electrolyte and under pressure this mixture is forced to flow into the gap between the workpiece and the electrode tool. Kimoto et al. discloses that a clean mirror surface finish of not more than 0.5 μm Rmax may be obtained when the current density is not more than 2.5 A/cm2. The polishing time being one minute for such result. However, where current density is from 0.5 to 1 A/cm2, a polishing time of 3-5 minutes is required. Kimoto et al. further discloses that either an abrasive cloth or an abrasive buff may be used instead of the abrasive powder mixture of the electrolyte. Rmax is defined as the largest of the individual surface peak to valley from each sample length.
Further attempts have been made to increase the accuracy of electrolytic machining. For example, Rhoades, U.S. Pat. No. 5,114,548, discloses a method of electrochemical machining a workpiece utilizing an electrode that is passive to the workpiece, wherein the conductive tools are provided with a non-conductive abrasive surface, and the tool and workpiece are brought together with a contacting relative motion so that the abrasive surface will selectively abrade the workpiece to remove any passivation layer therefrom in those areas to be machined and such that the unabraded surface areas will retain the passivation layer to prevent electrochemical machining thereof. Reciprocal motion between the tool and workpiece is also effective to pump the electrolyte through the gap between the tool and workpiece and prevent the workpiece from becoming overheated. Orbital abrading is a nontraditional machining process which is entirely different and distinct from electrochemical machining. Unlike conventional grinding techniques, orbital abrading utilizes a very small relative movement having a radius of orbit, typically of 0.020 to 0.100-inch at a typical rate, of 1200 oscillations per minute. Because of the very small orbital displacement of the workpiece and tool during working, the difference in size is small, and further, permits production of rather detailed and intricate ground configurations with a high degree of resolution, either two or three dimensional forms.
While orbital abrading is a well accepted nontraditional machining process based solely on its own merits, there are other known machining processes wherein orbital abrading, or at least orbital motion between the tool and workpiece, has been combined with other machining techniques. For example, U.S. Pat. No. 3,593,410 issued to Taylor, teaches a machining process which utilizes vibratory motion between an abrasive tool and workpiece wherein the interface is submerged in a solution which will chemically alter the workpiece surface to facilitate a mechanical abrasion. U.S. Pat. No. 3,663,786, issued to O'Connor, teaches an electrical discharge machine tool which provides an abrasive relative motion between the electrode and the workpiece, primarily for the purpose of machining graphite electrical discharge machining electrodes. Of more relevance to this invention, U.S. Pat. No. 3,564,190, issued to Kandajan et al. teaches a number of machining processes including an electrochemical machining process wherein a relative motion, orbital or otherwise is imparted between the tool and workpiece, so that the workpiece is machined by the combined activity of the two processes. This reference does not, however, teach or suggest the unique features of this invention as essential to affect the greater degree of precision, but rather, relies merely on the combined metal removal technique to speed the machining process. Further, the use of a tool with a surface covered with abrasive particles can result in the embedding of abrasive particles into the surface of the article being processed, and this factor can detract from the surface quality of the processed workpiece when an abrasive coated tool is used as a final step in the electrochemical machining process.